1. Field of the Invention
The present invention relates to a reactive sputtering method and a reactive sputtering apparatus which provide an excellent film quality stability.
2. Description of the Related Art
For realizing a highly functional digital device, it is indispensable to develop a memory to be used having a higher capacity, a higher speed, a lower power consumption, a longer lifetime, and the like. Especially, a flash memory is used for various applications and expected to have a further higher performance. However, a flash memory using a floating gate, which is mainstream at present, has a problem that a threshold voltage variation is caused by interference through a capacitive coupling between memory cells neighboring each other along with miniaturization of the memory cell and it is generally known that there exists a limit for the miniaturization.
Hence, a device drawing attention for replacing the flash memory is a ReRAM which is provided with a metal oxide and has a recording principle suitable for the miniaturization. ReRAM is an abbreviation of Resistivity Random Access Memory, and the ReRAM is a nonvolatile memory which can be caused to change a state (specifically, resistance value) of metal oxide with a pulse voltage and can preserve the information unless a pulse voltage is applied again. Further, the ReRAM can reduce cost utilizing the simplicity of the device structure and operation, and is considered to be operated even in an order of 50 ns or less, and thereby various ideas are being proposed using this device.
For the resistance change layer of the ReRAM, there are used a perovskite material such as PrCaMnO3 (PCMO), LaSrMnO3 (LSMO) and GdBaCoxOy (GBCO), and a two-element type transition metal oxide material which has a composition shifted from a stoichiometric one, such as nickel oxide (NiO), vanadium oxide (V2O5), zinc oxide (ZNO), niobium oxide (Nb2O5), titanium oxide (TiO2), cobalt oxide (CoO), tantalum oxide (Ta205) and tungsten oxide (WO2).
A means for fabricating a metal compound such as the metal oxide layer and the like includes reactive sputtering which performs sputtering of a metal target using reactive gas such as oxygen gas and nitrogen gas, and the process having an extinguished controllability and reproducibility is considered to be required for the production.
When performing continuous film deposition using the reactive sputtering, however, there arises a problem that a film characteristic is different for each processing. This phenomenon is shown in FIG. 8. The data of FIG. 8 shows a specific resistance change of a film (here, Ta oxide) deposited on a substrate against the number of times of processing when the processing is performed by oxygen reactive sputtering of a metal target Ta using a DC magnetron sputtering apparatus. This data shows that the specific resistance increases as the number of times of processing increases and it is found that the specific resistance increases in 26% from the first time to the 50th time.
The cause of the specific resistance increase includes that an oxygen gas amount taken-in (gettered) by a metal compound adhering to a shield provided in a sputtering apparatus changes depending on a case. The reason that the amount of gettered oxygen gas changes is considered to be a temperature change of the shield. The surface temperature of the shield is low while the number of times of processing is small and a degassing amount (ejected gas amount) from the metal compound adhering to the shield is small, and thereby the oxygen gettering effect is large in the metal compound. On the other hand, as the number of times of processing increases, the shield accumulates plasma heat and the shield temperature increases due to the accumulated heat, and then the degassing amount increases. The oxygen gettering effect decreases gradually as this degassing amount increases, resulting in the increase of the specific resistance along with the increase of the number of times of processing.
As a means for suppressing the change of the reactivity for each number of times of processing, there is a method of performing a dummy run before the continuous processing for a sufficiently long time until the shield comes to have a temperature which is to be reached during the sputtering process. This method, however, results in a shorter target shield life and a reduced throughput, and does not provide a sufficiently effective countermeasure.
Further, Japanese Patent Laid-Open No. H5-175157 proposes to heat the shield (200° C. to 500° C.) preliminarily using heater heating, gas heating or the like. This proposal intends to stabilize the reactivity by realizing a thermal equilibrium state preliminarily within a sputtering chamber and to suppress a thermal variation during the sputtering process. However, the inside of the chamber is heated to 200° C. or more and thereby the deposition cannot be carried out in a sufficiently cooled atmosphere and further it takes a long time until the shield surface reaches a thermal equilibrium similarly to the above case. Further, when using a material in which a crystal state changes between at a low temperature and at a high temperature such as Al oxide (γ-alumina, α-alumina, or the like) or a material which forms various coupling states with oxygen such as Ta oxide (TaO2, Ta2O5, or the like), it is difficult to control the reaction precisely in the film deposition by the above method.